In an aircraft or a coach where passengers are always involved with noises, the passengers in the seats sometimes cannot clearly catch information provided through audio, such as an in-flight notice, due to the noises around the seats.
The aircraft or the coach defines an interior space with continuous walls, so that the interior space forms a kind of enclosed structure. If noise sources exist inside and outside the interior space, the passengers in the interior space are to be confined within a regular noise environment. An excess noise sometimes invites physical or mental stress to the passengers, thereby degrading the convenience in the interior space. In the case of an aircraft, in particular, although flight attendants try to provide the passengers with good service in the interior space, this degradation in convenience becomes a critical problem to a service quality.
In the case of the aircraft, the following noises are chiefly involved: noises produced by the devices such as a propeller or an engine which generates thrust force for the aircraft, and noises, such as zip sound generated by the nose and the wings of the aircraft, involved with airstream produced by the movement of the aircraft in the air. The foregoing noises audible in the interior space make the passengers unpleasant and also hinder the in-flight audio notice. The noises thus need to be reduced.
Passive attenuating measures have been taken, in general, for reducing the noises in the enclosed space. This method places sound insulating material, such as a diaphragm or sound absorption material, between the enclosed structure and the noise source. The diaphragm includes, e.g. a high density diaphragm, and the sound absorption material includes, e.g. a sound absorption sheet. However, the acoustic absorption material has a high density and thus becomes a weight-gaining coefficient. An increment in the weight consumes a greater amount of fuel or reduces a flight range. As a result, the increment in the weight incurs degrading the economical and functional performances of the aircraft. On top of that, the foregoing materials have a problem of strength such as being subject to damages and a problem of design such as having a poor quality image.
Active attenuating measures have been taken for overcoming the foregoing problems caused by the passive attenuating measures, for instance, a method of generating an acoustic wave having an opposite phase to that of the noise is used generally for noise reduction. This method allows reducing the noise at the noise source or around the noise source, thereby preventing the noise from propagating to a region where noise reduction is needed. To be more specific, a noise reduction device described below has been proposed:
The noise reduction device comprising:
                a microphone for sensing a sound generated by a noise source;        a controller for amplifying an electric signal supplied from the microphone and then reversing a phase of the electric signal; and        a speaker for converting the electric signal supplied from the controller into sound and then outputting the sound.This device is disclosed in Patent Literature 1.        
To find an acoustic transmission function from a speaker to a noise controlling point is needed for designing an active noise reduction device. The transmission function is measured, in general, this way: A white noise having a flat frequency characteristic in a frequency control band is generated from the speaker, and a microphone placed at the control point senses this white noise. At this time, an external noise level is measured, and the white noise of which level is higher than the external noise level by a given amount, e.g. 10 dB, should be generated. This method is disclosed in Patent Literature 2.
The method disclosed in Patent Literature 2 can be used only in a case where one noise reduction device is installed, and the acoustic transmission function between the speaker and the microphone placed at the control point can be measured by the foregoing method. Patent Literature 2, however, keeps silent about a case where multiple noise reduction devices are installed. In an airplane, each one of seats is equipped with a noise reduction device, so that multiple noise reduction devices, i.e. in a quantity equal to the number of seats, need the acoustic transmission functions. In such a case, it is desired to measure fast the acoustic transmission functions of the respective seats, i.e. the noise reduction devices, free from being affected by external noises.